Crystal structure of a water oxidation catalyst solvate with composition (NH4)2[FeIV(L-6H)]·3CH3COOH (L = clathrochelate ligand)

The coordination polyhedron of the complex [FeIV(C12H12N12O6)]2− or [FeIV(L–6H)]2− anions exhibits a shape intermediate between trigonal-prismatic and antiprismatic.

The synthetic availability of molecular water oxidation catalysts containing highvalent ions of 3d metals in the active site is a prerequisite to enabling photo-and electrochemical water splitting on a large scale.Herein, the synthesis and crystal structure of diammonium {�-1,3,4,7,8,10,12,13,16,17,19,22-dodecaazatetracyclo[8.8.4.1 3,17 .1 8,12 ]tetracosane-5,6,14,15,20,21-hexaonato}ferrate(IV) acetic acid trisolvate, (NH 4 ) 2 [Fe IV (C 12 H 12 N 12 O 6 )]•3CH 3 COOH or (NH 4 ) 2 [Fe IV (L-6H)]•3CH 3 COOH is reported.The Fe IV ion is encapsulated by the macropolycyclic ligand, which can be described as a dodeca-aza-quadricyclic cage with two capping triazacyclohexane fragments making three five-and six sixmembered alternating chelate rings with the central Fe IV ion.The local coordination environment of Fe IV is formed by six deprotonated hydrazide nitrogen atoms, which stabilize the unusual oxidation state.The Fe IV ion lies on a twofold rotation axis (multiplicity 4, Wyckoff letter e) of the space group C2/c.Its coordination geometry is intermediate between a trigonal prism (distortion angle ' = 0 � ) and an antiprism (' = 60 � ) with ' = 31.1 � .The Fe-N bond lengths lie in the range 1.9376 (13)-1.9617(13) A ˚, as expected for tetravalent iron.Structure analysis revealed that three acetic acid molecules additionally cocrystallize per one iron(IV) complex, and one of them is positionally disordered over four positions.In the crystal structure, the ammonium cations, complex dianions and acetic acid molecules are interconnected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors.

Chemical context
The design of robust and efficient water oxidation catalysts based on 3d metals requires a rational approach that considers both their redox properties and crystal structure (Blakemore et al., 2015).The intrinsic lability of the M-L bonds (M = central 3d metal cation, L = ligand) in aqueous solution is one of the main design challenges (Gil-Sepulcre & Llobet, 2022).In addition, the ligand in the catalyst has to be simple and oxidatively robust, otherwise it will be oxidized in the course of the catalysis (Boniolo et al., 2022).Efficient chemical (Shylin et al., 2019a) and photochemical (Shylin et al., 2019b) water splitting using a clathrochelate complex Na 2 [Fe IV (L-6H)] as a catalyst has recently been reported.The relatively high reaction rate and turnover number have been attributed to the exceptional stability of this cage compound bearing the Fe ion in the unusual oxidation state +IV.Clathrochelate complexes [Fe IV (L-6H)] 2À with various cations (hexamethylenetetraminium, Bu 4 N + , Ph 4 As + , [Ca(H 2 O) 2 ] 2+ , Li + ) have been obtained and characterized structurally and spectroscopically (Tomyn et al., 2017;Plutenko et al., 2023).The Fe IV ion can be reduced to Fe III or oxidized to Fe V , either chemically or electrochemically, but at ambient conditions it spontaneously returns to the Fe IV state in air, showcasing the stability of the oxidation state +IV in this specific ligand environment.Related compounds with [Mn IV (L-6H)] 2À clathrochelate anions have also been described recently (Shylin et al., 2021).
In this communication, we report on the template synthesis and crystal structure of the co-crystal compound (NH 4 ) 2 [Fe IV (C 12 H 12 N 12 O 6 )]•3CH 3 COOH or (NH 4 ) 2 [Fe IV (L-6H)]•-3CH 3 COOH, which was obtained in an attempt to explore alternative crystallization strategies of cage compounds.We thus demonstrate that Fe IV clathrochelates can be obtained in the form of single crystals under mild conditions.

Structural commentary
The title compound consists of two ammonium cations, a clathrochelate dianion [Fe IV (L-6H)] 2À , and three co-crystallized acetic acid molecules per one formula unit (Fig. 1).The core of the macrocyclic ligand L is the hexahydrazide N-donor cage capped by two 1,3,5-triazacyclohexane fragments, thus featuring three five-and six six-membered chelate rings.All six hydrazide groups are deprotonated, and the formal charge of the ligand (L-6H) is 6-.The cage encapsulates the Fe ion in the oxidation state +IV, stabilized by the strong �-donor capacity of the ligand (L-6H), as well as its ability to shield the ion from external factors.The shape of the coordination polyhedron [Fe IV N 6 ] cannot be described as octahedral, which is typical for most ferrous and ferric complexes.It is rather intermediate between an ideal trigonal prism (' = 0 � ) and an antiprism (' = 60 � ) with an average ' = 31.1 � , calculated as a mean rotation of the N1-N3-N5 triangular base relative to N1 i -N3 i -N5 i .It is within the range of 28.0-32.3� reported for other Fe IV and Mn IV clathrochelates (Tomyn et al., 2017;Shylin et al., 2021).
One of three acetic acid molecules is disordered, leading to four equivalent positions (Fig. 1).Specifically, the two C atoms of CH 3 COOH are disordered along the C-C bond -each can serve as either a methyl or a carboxyl C atom.They are additionally disordered between two positions each by means of the twofold symmetry axis.As such, occupancy factors of C and O atoms are 0.5 and 0.25, respectively.

Supramolecular features
In the crystal structure of the title compound, the ammonium cations; complex anions and acetic acid molecules are associated via an intricate set of O-H� � �O, N-H� � �O, N-H� � �N, and non-classical C-H� � �O hydrogen bonds (Table 2).Most of these contacts show angles far from linearity, indicating that they correspond to rather weak interactions.However, a few of them can be considered as significant intermolecular contacts and are discussed in more detail.Each clathrochelate anion appears to be associated with two CH 3 COOH co-crystallized molecules and four NH 4 + cations, thus employing all six oxamide O atoms as acceptors for hydrogen bonding.The oxamide ribs of the clathrochelate exhibit different binding modes.Specifically, the (O1,O3) ribs are bound to CH 3 COOH and NH 4 + through the O8-H8� � �O1 i and N13-H13E� � �O3 iii contacts, while the (O5,O5 0 ) ribs are bound to two NH 4 + ions through the crystallographically equivalent N13-H13F� � �O5 contacts (Fig. 2).The latter contacts are somewhat weaker than the former (note their D� � �A distances and angles, Table 2), which creates higher distortion of the oxamide moieties O1-C1-C3 i -O3 i in favor of virtually linear hydrogen bonds.The non-protonated O atom of CH 3 COOH serves as an acceptor for another NH 4 + proton, making N13-H13D� � �O7 ii contacts.The fourth remaining proton of NH 4 + is involved in binding the neighboring clathrochelate anion through the N13-H13G� � �O1 iv contact.
All in all, the NH 4 + cations, isolated complex anions and cocrystallized CH 3 COOH are connected into a tri-periodic supramolecular framework by means of hydrogen bonds, mainly via oxamide O atoms as proton acceptors, and NH 4 + and CH 3 COOH as donor groups.   of these structures represent mononuclear complexes with Bu 4 N + and Ph 4 As + cations (Tomyn et al., 2017), and four are coordination polymers in which Ca 2+ (Tomyn et al., 2017), Mn 2+ (Xu et al., 2020a), or Cu 2+ (2 structures; Xu et al., 2020b) cations are exo-coordinated to the vacant (O,O 0 ) and/or (O,N) chelating units of the hexahydrazide ligand.To the best of our knowledge, there has been only one structure of the Fe IV hexahydrazide complex reported after November 2022 (Plutenko et al., 2023).

Synthesis and crystallization
A powder of (Bu 4 N) 2 [Fe IV (L-6H)] was obtained by a metal template synthesis as described previously (Tomyn et al., 2017).Then, 0.5 mmol of (Bu 4 N) 2 [Fe IV (L-6H)] and 1 mmol of CH 3 COONH 4 were dissolved in 10 ml of water, and 10 ml of glacial acetic acid was added to this mixture.The resulting mixture was evaporated under vacuum on a rotary evaporator to a volume of ca 10 ml and left in a closed flask.After two weeks, dark-green crystals of (NH 4 ) 2 [Fe IV (L-6H)]•-3CH 3 COOH suitable for the X-ray diffraction analysis were obtained.FTIR (in KBr pellet, cm À 1 ): 3424 (O-H), 3184 (N-H), 2953 (C-H), 1636 (C O, amide I).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.The H atoms attached to C were placed in fixed idealized positions using a riding model with U iso (H) = 1.2U eq (C) for methylene and 1.5 for methyl groups.
The non-disordered H atoms attached to N and O were located in difference-Fourier maps and their positional parameters were verified according to the hydrogen-bonding geometry.Occupancy factors of C and O atoms of the disordered CH 3 COOH molecule were fixed to 0.5 and 0.25, respectively.The H atoms attached to disordered O atoms were placed in fixed positions with U iso (H) = 1.5U eq (O), and their coordinates were refined according to the riding model described above.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Figure 2
Figure 2 The crystal packing in (NH 4 ) 2 [Fe IV (l-6H)]•3CH 3 COOH.Relevant hydrogen bonds are shown as dashed lines.The disordered CH 3 COOH molecule is shown with only one possible orientation.Color code: Fe, black; N, blue; O, red; C, dark ray; H, gray; the unit-cell is outlined.

Table 3
Experimental details.